Motor control topology for airborne power generation and systems using same

ABSTRACT

In an example embodiment, a system includes a plurality of drive units coupled to a plurality of propellers. Each drive unit includes a single motor/generator and a single motor controller, and the plurality of drive units includes a first drive-unit pair and a second drive-unit pair. The system also includes a high-voltage bus connecting the motor controllers in the first drive-unit pair to a tether, a low-voltage bus connecting the motor controllers in the second drive-unit pair to the tether, and an intermediate-voltage bus connecting the motor controllers of the first drive-unit pair in series with the motor controllers of the second drive-unit pair. The motor controllers in the first drive-unit pair are connected in parallel via the high-voltage bus and the intermediate-voltage bus, and the motor controllers in the second drive-unit pair are connected in parallel via the intermediate-voltage bus and the low-voltage bus.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a continuation patent application of U.S.application Ser. No. 14/154,004, filed Jan. 13, 2014, the contents ofwhich are entirely incorporated herein by reference, as if fully setforth in this application, which claims priority to U.S. ProvisionalPatent Application No. 61/751,817, filed Jan. 11, 2013, the contents ofwhich are entirely incorporated herein by reference, as if fully setforth in this application.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Power generation systems may convert chemical and/or mechanical energy(e.g., kinetic energy) to electrical energy for various applications,such as utility systems. As one example, a wind energy system mayconvert kinetic wind energy to electrical energy.

SUMMARY

A wind energy system may take the form of an airborne wind turbine (AWT)system. AWT systems may extract useful power from the wind for variouspurposes such as, for example, generating electricity, lifting or towingobjects or vehicles, etc.

In an AWT system, it may be desirable to achieve a high operatingvoltage so as to, for example, reduce ohmic losses in the AWT system.While the operating voltage of a single motor in an AWT may be limitedby practical electronics, a higher operating voltage may be achieved bystacking motor/generator in series. (Herein, a “motor/generator” may bea component that can switch between functioning as a motor for apropeller, and a generator that converts kinetic energy of the rotatingpropeller to electrical energy.)

However, motors that are stacked in series may exhibit instability. Morespecifically, if motor/generators that are stacked in series arecontrolled to have approximately equal power, and the respectivecurrents running through each motor/generators are not approximatelyequal, the voltages of the motor/generators may diverge, such that themotor/generators exhibit voltage instability. Disclosed herein areconfigurations for motor/generators that are stacked in series in anAWT, and controls systems therefore. In particular, an example aerialvehicle in an AWT may include pairs of drive units may be stacked inseries, with each drive unit in a drive-unit pair including amotor/generator and a motor controller. Beneficially, some examplesdescribed herein may allow for high operating voltages on the tether tobe connected to a number of lower voltage drive units, while reducing oreliminating the risk of voltage instability.

In one aspect, a system includes: (a) an aerial vehicle comprising aplurality of propellers; (b) a plurality of drive units coupled to theplurality of propellers, wherein each drive unit comprises amotor/generator and a motor controller, wherein the plurality of driveunits comprises at least two pairs of drive units comprising a firstdrive-unit pair and a second drive-unit pair, wherein the drive units ineach pair of drive units are connected in parallel, and wherein the atleast two pairs of drive units are connected in series; and (c) atether, wherein the tether configured to transmit power down from theplurality of motor/generators when the drive units operate in a firstmode, and to transmit power up to the plurality of motor/generators whenthe drive units operate in a second mode. Further, the motor controllersin the drive units are connected to the tether in parallel and inseries.

In another aspect, a system includes: (a) a plurality of drive unitsthat are couplable to a plurality of propellers, wherein each drive unitcomprises a motor and a motor controller, wherein the plurality of driveunits comprises at least two pairs of drive units comprising a firstdrive-unit pair and a second drive-unit pair, wherein the drive units ineach pair of drive units are connected in parallel, and wherein the atleast two pairs of drive units are connected in series; (b) a first busthat connects the motor controllers in at least the first drive-unitpair to a tether; (c) a second bus that connects the motor controllersin at least the second drive-unit pair to the tether; and (d) a thirdbus that connects the motor controllers in the first and seconddrive-unit pairs in series.

In a further aspect, a system includes: (a) an aerial vehicle comprisinga plurality of propellers; (b) a plurality of drive units coupled to theplurality of propellers, wherein each drive unit comprises a pluralityof motor/generators and a corresponding plurality of motor controllerswherein the plurality of drive units comprises at least two pairs ofdrive units comprising a first drive-unit pair and a second drive-unitpair, wherein the drive units in each pair of drive units are connectedin parallel, and wherein the at least two pairs of drive units areconnected in series; (c) a tether, wherein the tether configured totransmit power down from the plurality of motor/generators when thedrive units operate in a first mode, and to transmit power up to theplurality of motor/generators when the drive units operate in a secondmode. Further, the motor controllers in the drive units are connected tothe tether in parallel and in series.

These as well as other aspects, advantages, and alternatives will becomeapparent to those of ordinary skill in the art by reading the followingdetailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting a kite system according to someembodiments of the present invention.

FIG. 2 is an illustration of a kite according to some embodiments of thepresent invention.

FIG. 3 is a diagram depicting an electrical system according to someembodiments of the present invention.

FIG. 4 is a diagram depicting a simplified equivalent circuit of anelectrical system according to some embodiments of the presentinvention.

FIG. 5 is a diagram depicting a motor/generator according to someembodiments of the present invention.

FIG. 6 is an illustration of an airborne power generation systemaccording to some embodiments of the present invention.

FIG. 7 is a diagram depicting the thrust and drag curves.

DETAILED DESCRIPTION

Example methods and systems are described herein. It should beunderstood that the words “example,” “exemplary,” and “illustrative” areused herein to mean “serving as an example, instance, or illustration.”Any embodiment or feature described herein as being an “example,” being“exemplary,” or being “illustrative” is not necessarily to be construedas preferred or advantageous over other embodiments or features. Theexample embodiments described herein are not meant to be limiting. Itwill be readily understood that the aspects of the present disclosure,as generally described herein, and illustrated in the figures, can bearranged, substituted, combined, separated, and designed in a widevariety of different configurations, all of which are explicitlycontemplated herein.

Overview

Illustrative embodiments relate to aerial vehicles, which may be used ina wind energy system, such as an Airborne Wind Turbine (AWT). Inparticular, motor topologies relevant to aerial vehicles in AWTs andcontrol processes for such motor control topologies are disclosedherein.

By way of background, an AWT may include an aerial vehicle that flies ina path, such as a substantially circular path, to convert kinetic windenergy to electrical energy. In an illustrative implementation, theaerial vehicle may be connected to a ground station via a tether. Whiletethered, the aerial vehicle can: (i) fly at a range of elevations andsubstantially along the path, and return to the ground, and (ii)transmit electrical energy to the ground station via the tether. (Insome embodiments, the ground station may transmit electricity to theaerial vehicle for take-off and/or landing.)

In an AWT, an aerial vehicle may rest in and/or on a ground station (orperch) when the wind is not conducive to power generation. When the windis conducive to power generation, the ground station may deploy (orlaunch) the aerial vehicle. For example, in one embodiment, the aerialvehicle may be deployed when the wind speed is at or greater than 3.5meters per second (m/s) at an altitude of 200 meters (m), In addition,when the aerial vehicle is deployed and the wind is not conducive topower generation, the aerial vehicle may return to the ground station.

Moreover, in an AWT, an aerial vehicle may be configured for hoverflight and crosswind flight. Crosswind flight may be used to travel in amotion, such as a substantially circular motion, and thus may be theprimary technique that is used to generate electrical energy. Hoverflight in turn may be used by the aerial vehicle to prepare and positionitself for crosswind flight. In particular, the aerial vehicle couldascend to a location for crosswind flight based at least in part onhover flight. Further, the aerial vehicle could take-off and/or land viahover flight.

In hover flight, a span of a main wing of the aerial vehicle may beoriented substantially parallel to the ground, and one or morepropellers (or rotors) of the aerial vehicle may cause the aerialvehicle to hover over the ground. In some embodiments, the aerialvehicle may vertically ascend or descend in hover flight.

In crosswind flight, the aerial vehicle may be propelled by the windsubstantially along a path, which as noted above, may convert kineticwind energy to electrical energy. In some embodiments, the one or morepropellers of the aerial vehicle may generate electrical energy byslowing down the incident wind.

The aerial vehicle may enter crosswind flight when (i) the aerialvehicle has attached wind-flow (e.g., steady flow and/or no stallcondition (which may refer to no separation of airflow from an airfoil))and (ii) the tether is under tension. Moreover, the aerial vehicle mayenter crosswind flight at a location that is substantially downwind ofthe ground station.

In some embodiments, a tension of the tether during crosswind flight maybe greater than a tension of the tether during hover flight. Forinstance, in one embodiment, the tension of the tether during crosswindflight may be 15 kilonewtons (KN), and the tension of the tether duringhover flight may be 1 KN.

In line with the discussion above, an example aerial vehicle maygenerate electrical energy in crosswind flight and may thereby allow theAWT to extract useful power from the wind. The aerial vehicle maygenerate electrical energy in various wind conditions such as high windspeeds, large gusts, turbulent air, or variable wind conditions.Generally, the inertial speed of the aerial vehicle, the tension of thetether, and the power output of the AWT increase as the wind speedincreases. Additionally, the power output typically has a maximumeffective limit (rated power output). The wind speed at which the poweroutput limit is reached is defined as the rated wind speed.Additionally, the power generation components of the AWT may produceheat, and as power output increases, the heat production may increase,potentially limiting the operational parameters of the AWT. Therefore,it may be desirable to enact control schemes that control the powergeneration components and therefore control their heat production.

Considering this, disclosed embodiments may allow for operating anaerial vehicle of an AWT in crosswind-flight in a manner that mayefficiently generate power generation in variable wind conditions suchas those noted above. Further, while there are other motorconfigurations capable of achieving a high voltage at the tether, suchas a voltage converter converter or the use of another intermediate lowto high voltage conversion device, the embodiments described herein maybe advantageous over other solutions by using only a single stage toconnect low-power motor controllers to a high-voltage tether, which inturn may lower cost and increase the efficiency with which the AWTconverts wind energy to electrical energy.

Illustrative Airborne Wind Turbines

FIG. 1 is a diagram depicting an embodiment of a AWT system 100 inaccordance with example embodiments. Herein, an AWT system, such as AWTsystem 100, may also be referred to as a “kite system,” and the aerialvehicle 101 in an AWT system 100 may also be referred to as a “kite.”

The kite system 100 comprises a tether 103 that connects the kite 101 tothe ground station 102. The kite 101 may fly along the flightpath 104 inorder to generate power; e.g., by converting wind energy to electricalenergy. In an example embodiment, kite 101 may fly along flightpath 104at a high multiple of of the speed of the wind 122.

The aerial vehicle 101 may also be referred to as “kite 101,” and mayinclude or take the form of various types of devices, such as a wing,and/or an airplane, among other possibilities. The aerial vehicle 101may be formed of solid structures of metal, plastic and/or otherpolymers. The aerial vehicle 101 may be formed of any material whichallows for a high thrust-to-weight ratio and generation of electricalenergy which may be used in utility applications. Additionally, thematerials may be chosen to allow for a lightning hardened, redundantand/or fault tolerant design which may be capable of handling largeand/or sudden shifts in wind speed and wind direction. Other materialsmay be possible as well.

The ground station 102 may be used to hold and/or support the kite 101until it is in an operational mode. The ground station 102 may also beconfigured to allow for the repositioning of the aerial vehicle 101 suchthat deploying of the device is possible. Further, the ground station102 may be configured to receive the aerial vehicle 101 during alanding. The ground station 102 may be formed of any material that cansuitably keep the aerial vehicle 101 attached and/or anchored to theground while in hover flight, forward flight, crosswind flight. In someimplementations, a ground station 102 may be configured for use on land.However, a ground station 102 may also be implemented on a body ofwater, such as a lake, river, sea, or ocean. For example, a groundstation could include or be arranged on a floating off-shore platform ora boat, among other possibilities. Further, a ground station 102 may beconfigured to remain stationary or to move relative to the ground or thesurface of a body of water.

In some embodiments, to launch and land, the kite 101 hovers underthrust from the rotors 109, which are controlled by a control system. Inexample embodiments, the control system may be automated (e.g., operatewithout a human operator) or be partially automated. Further, thecontrol system may implement control of the kite 101, at least in part,by controlling drive units for the rotors. Each drive unit may include amotor controller and one or more motors connected the propellers via ashaft. The motor controller may be operable to adjust the amount and/ordirection of current between the motor a circuit to which the drive unitis connected (e.g., a circuit with the tether 103). Accordingly thecontrol system may send control signals to the motor controllers inorder to control the kite 101.

To provide pitch control authority for kite 101, the rotors 109 may bedistributed both substantially above and below the center of gravity ofthe kite 101, as viewed in the typical aircraft build reference frame.In some embodiments, the tail 106 may include a horizontal element whichis located substantially above the kite center of gravity, and which maybe configured to rotate 90 degrees to pitch down during hover flight,which helps to both reduce the pitching moment on the kite 101 due tothe wind 122 on the tail 106, and to stabilize the kite 101 in pitch.

The kite 101 further comprises a main wing 105. In embodiments in whichkite system 100 is used to generate power, the main wing 105 is used togenerate substantial lift, such that the kinetic energy available in thewind 122 is transferred into the kite 101 in the same manner as the tipof a wind turbine blade. However, a subset of the rotors 109 may belocated substantially above the wing 105 to provide pitch stabilityduring hover flight, and to reduce the interaction of the wake of therotors 109 with the main wing 105.

FIG. 2 is a diagram depicting an example aerial vehicle 201 in greaterdetail. As with aerial vehicle 101, aerial vehicle 201 may also bereferred to as kite 201. Kite 201 may be implemented as the kite 101 inthe kite system 100 depicted in FIG. 1, and/or may be implemented inother systems, such as other AWTs.

The kite 201 comprises a main wing 205, which generates substantial liftduring operation of the kite 201 along a flightpath (e.g. flightpath104). In some embodiments, the main wing 205 comprises a trailingelement 207, which increases the maximum coefficient of lift which maybe generated by the main wing 205.

The kite 201 further comprises a tail 206, which may serve both tocounter the pitching moment generated by the main wing 205 and trailingelement 207, and to increase the pitch, yaw, and coupled stability ofthe kite 201. However, the tail 206 adds mass to the kite 201.Therefore, in order to locate the kite's center of mass 223 at a targetlocation near the quarter chord of the main wing 205, a countering massmay be located forward of the wing 205. For example, rotors 209, alongwith the motor/generators (not shown) that drive rotors 209 may belocated towards the front of the main wing 205 such that the mass of therotors 209 and the motor/generators counters the mass of the tail 206,resulting in a center of mass 223 at the desired location.

The rotors 209 and the associated motor/generators are attached to thewing through the motor pylons 212. In the illustrated embodiment, themotor pylons 212 locate the rotors 209 in a pattern with rotorssubstantially above and/or below the main wing, such that the averagerotor location has a substantial distance from any particular rotorlocation, as viewed on the plane perpendicular to kite direction offlight. In some embodiments, such as that shown in FIG. 2, the rotorsare aligned in two horizontal rows of rotors. In other embodiments, therotors may be arranged in an octagonal pattern, hexagonal pattern, oranother pattern.

FIG. 3 is a diagram depicting the electrical system for an airborne windturbine that, for the purposes of explanation, comprises four driveunits, 321, 322, 323 and 324. Such an electrical system could compriseas few as two or as many as twelve, twenty, or possibly even more moredrive units. In some applications, four drive units is the smallestnumber of drive units that will allow for different power levels to besourced from or sunk to each motor/controller without unduly increasingthe voltage across each drive unit. An example embodiment includes eightdrive units. However, more or less drive units are possible, withoutdeparting from the scope of the invention.

In an example embodiment, each drive unit 321, 322, 323, and 324 mayinclude a propeller 309, a motor/generator 308, and a motor controller307. Alternatively, a “drive unit” may be considered to include amotor/generator 308 and a motor controller 307, and the propeller 309may be considered separate a component from the drive unit, which iscoupled to the motor/generator in the drive unit.

Note that the motor controllers 307 of drive units 321 and 322 areconnected in parallel. As such, drive units 321 and 322 may becollectively referred to as “pair” of drive units (i.e., a “drive-unitpair”). Similarly, because the motor controllers 307 of drive units 323and 324 are connected in parallel, drive units 323 and 324 may also becollectively referred to as a drive-unit pair. In a further aspect,according to an example motor topology, the drive-unit pairs areconnected in series. For example, in the illustrated embodiment, a firstdrive-unit pair (drive units 321 and 322) is connected in series with asecond drive-unit pair (drive units 323 and 324).

The motor controllers 307 may take the form of or include switchinghardware that is operable to control the current and/or phase of currentthrough the windings of motor/generators 308. In some embodiments, themotor controllers 307 may utilize power switching devices such asinsulated-gate bipolar transistor (IGBT) switches ormetal-oxide-semiconductor field-effect transistor (MOSFET) switches thatare etched into silicon or other semiconductor dies. The maximumattainable voltage between any two leads of a power switching device istypically limited, which in turn limits the maximum attainable voltagewithin a circuit utilizing the power switching device. Furthermore, thedelay time when switching a power switching device from an “on” state toan “off” state typically increases significantly as the maximum voltageof the switching device increases. When not fully “on” or “off,” powerswitching devices typically have a significant voltage drop betweenleads, while also conducting current, thus resulting in significantinternal power dissipation by the power switching devices.

The motor/generators 308, which could be the motors/generators 208 ofFIG. 2, are attached mechanically to the propellers 309 in such a mannerthat the propellers 309 convey a torque to the shaft of the motor 308.The stator of the motor 308 is mounted to the airframe of the airbornewind turbine (which could be airborne wind turbine 201), along with allother components encircled by bounding box 301. The motor controllers307 are connected electrically to the windings of motor/generators 308and commutate power from a high voltage DC bus 318 as appropriate todrive the current phases of the motor/generators 308.

In an example embodiment, the motor/generators 308 have three-phasewindings, while in other embodiments the motor/generators have someother number of phase windings, such as one-, two-, or six-phasewindings. The motor controllers 307 source the required power to theappropriate phases of the motor/generators at appropriate times so as toeither efficiently convert mechanical shaft power into electrical poweror convert electrical power into mechanical shaft power. The motorscontrollers 307 are connected in both parallel and series to the DC lineof the tether. Since the individual drive units in each drive-unit pairare connected to each other in parallel, the voltage at the intermediatevoltage DC bus 319 will be approximately half of the incoming voltage ontether 303, provided that the total power draw of the first drive-unitpair (drive units 321 and 322) is substantially equal to the total powerdraw of the second drive-unit pair (drive units 323 and 324).

Note that, for explanatory purposes, two drive-unit pairs are depictedin FIG. 3. In other embodiments, such as that which will be described inreference to FIG. 4, four pairs of drive units are connected in series,such that a total of eight drive units are each exposed to an average ofone quarter of the voltage between leads on the tether 303. Further, thenumber of drive-unit pairs may vary from these examples, withoutdeparting from the scope of the invention.

Each motor controller is connected to a high-side bus or a low-side bus,dependent in each case upon which drive unit the motor controller is acomponent of. The motor controller 307 of drive 321 is connected to thehigh voltage DC bus 318, which in turn is connected to the tether. Themotor controller 307 of drive 321 is also connected to the intermediatevoltage bus 319. The high voltage DC bus 318 may have any voltageacceptable for transfer of power over the tether 303, though for thepurpose of illustration a nominal voltage of 2000V is assumed. Ofcourse, other nominal voltages are possible, depending upon theparticular implementation. The voltage of the intermediate voltage DCbus 319 is dependent on the power draw of the drive units 321, 322, 323,and 324, and the voltage of the ground bus 320 is dependant on thevoltage drop along the conductors of the tether, relative to a referencezero voltage.

If the total power used by the drive units 321 and 322 is equal to thetotal power used by the drive units 323 and 324, the voltage across thedrive units 321 and 322 will be equal to the voltage across drive units323 and 324. Further, note that the voltage across the drive units 321and 322 is equal to the voltage between high voltage DC bus 318 andintermediate voltage DC bus 319, and that the voltage across drive units323 and 324 is equal to the voltage between the intermediate voltage DCbus 319 and the low voltage DC bus 320. Therefore, when the total powerused by the drive units 321 and 322 is equal to the total power used bythe drive units 323 and 324, the voltage between high voltage DC bus 318and intermediate voltage DC bus 319 will be equal to the voltage betweenhigh voltage DC bus 318 and intermediate voltage DC bus 319. Thus, inthis scenario, the voltage on the tether is equal to the voltage betweenhigh voltage DC bus 318 and low voltage DC bus 320, which is twice thevoltage across any single drive unit 321, 322, 323, or 324.

Consider an example where the voltage across the tether 303 is “high”;close to 2000V, for instance. In this example, the current which shouldbe carried by the tether is “low”; 300 amps on each of the twoconductors, for instance. Configured as such, the motor controllers 307can each operate on a nominal voltage of 1000V, and can thus expose thewindings of the motors 309 to lower peak voltage differences. Aslower-voltage transistors are typically more efficient and lower cost,the lower nominal voltage of the motor controllers 307 may help to allowfor lower-mass and/or lower-cost equipment.

In a further aspect, FIG. 3 shows a motor control system 330. Thecontrol system 330 may implement control logic for the drive-units 321,322, 323, and 324. In example embodiments, a motor control system, suchas control system 330, may implement control logic by controlling themotor controllers 307. In particular, the control system may sendcontrol signals to motor controllers 307 via a control-signal bus 332.Such control signals may instruct or cause a motor controller 307 toadjust the amount and/or direction of current through the correspondingmotor or motors in the same drive unit.

Further, control system 330 may be configured to receive, determine,and/or generate data that it can utilize to determine what controlsignals are appropriate. For example, control system 330 may receive orinformation from, or acquire information related to, the drive-units321, 322, 323, and 324, such as: (a) the voltage measured across eachdrive unit, and/or (b) the current through each drive unit (and/orthrough each phase leg of the motors). The control system 330 may alsoreceive other sensor data related to the kite 301. Control system 330may also use other types of data when applying a control scheme.

Yet further, control system 330 may be operable to send separate controlsignals to each motor controller 307, such that the motor/generator 308in each drive unit 321, 322, 323, 324 can be separately controlled.Thus, it should be understood that while FIG. 3 illustratescontrol-signal bus 332 with a single line, control system 330 may, inpractice, be separately wired to (or communicate over separate wirelesschannels with) each motor controller 307, such that different controlsignals can be sent to each motor controller 307.

Note that it may be beneficial for the tether 303 to have a small crosssection and a low mass. More specifically, a tether with a smaller crosssection may allow for better system performance, since a the smallercross section has less drag (and does not account for the majority ofdrag on the system) as the kite moves on its flightpath, such as whenkite 101 flies along the flightpath 104. The lower weight may reduce theamount of thrust needed in order for the kite 101 to hover, and may alsohelp to make the kite controllable. If the tether mass is high due tothick conductors, the dynamics of the tether will couple strongly withthose of the kite, and reduce the ability of the kite to turn precisely.Additionally, as the tether diameter and thus drag increases, theparasitic losses increase. This may in turn may increase the size of thekite that is required to produce a given amount of power, and thusincrease the loads. Thus, the use of a thin tether may be desirable.

The core of the tether comprises a light-weight high strength materialsuch as a carbon fiber pultrusion. In various embodiments, the core mayalso comprise dry kevlar fibers, a kevlar fiber composite, UHMWPE, orany other high strength material of comparatively low mass to lower coststructural elements (i.e., steel). The conductors of the tether comprisea high conductivity material, for example copper or aluminum. In anexample embodiment, the tether comprises a carbon fiber core andaluminum conductors. The conductors have a comparatively higher materialdensity (roughly 2700 kg/m″3 for aluminum, 9000 kg/mA3 for copper),suggesting that conductor mass has a much larger impact on tether massthan on tether cross section.

For the purpose of reducing tether diameter, and primarily of reducingtether mass, the airborne wind turbine may transmit power to and fromthe tether at a high voltage so as to reduce conductor cross sectionalarea. The density of insulators of the tether may be lower than that ofthe conductor for a range of conductors and insulators. In someembodiments, the conductor comprises aluminum and the insulationcomprises PVC. Furthermore, the desired volume of insulation per unittether length at higher voltages (1300 volts and above) may increase farmore slowly than the mass of conductor required at higher voltagesdecreases.

FIG. 4 is a diagram depicting a simplified equivalent circuit of anelectrical system for an airborne wind turbine, such as for the exampleelectrical system depicted in FIG. 3. The drive units 421, 422, 423, and424 comprise motor controllers, motors (e.g., motor/generators), andpropellers. In FIG. 4, the motors and propellers are represented by theinductances and resistances that are shown within the diagram of eachdrive unit 421, 422, 423, and 424. Drive units 421 and 422 are connectedbetween high voltage DC bus 418 and intermediate voltage DC bus 419,while the drive units 423 and 424 are connected between intermediatevoltage DC bus 419 and ground line 420. The combination of drive units421 and 422 is switched such that the combination of these two unitsdraws the same power as the combination drive units 423 and 424. Theremay be capacitors or low pass filters within or attached to each of thedrive units, which are configured to regulate voltage and/or delayresponse times of the voltage that is apparent on intermediate bus 419,such that control encompassing all four drive units is capable ofmaintaining the voltage on the intermediate bus 419 at a roughlyconstant level.

FIG. 5 is a diagram depicting an embodiment of a motor/generator, suchas the motor/generator 308 that is shown in FIG. 3. In particular, theelectromechanical component a motor/generator is depicted from a frontview, as projected along the axis of rotation. The motor/generatorcomprises stator iron 530 with teeth, such that coils of wire can bewound about the teeth. The motor/generator also comprises rotor iron532, to which magnets 533 are bonded. As the rotor rotates relative tothe stator, the alternating magnetic field produced by the magnets 533produces a change in flux through the coils about the teeth of stator530, generating back-EMF or the flow of electrical current. Due to theback-EMF induced along the wire of the coils 534, a voltage developsbetween different points along the length of wire.

In some embodiments, the motor/generator is controlled by a pulse-widthmodulating motor controller, such as the motor controller 307 depictedin FIG. 3. In such embodiments, the voltage difference across with wirein coils 534 is equal or nearly equal to the voltage on the bus thatconnects the motor controller and the motor/generator (e.g., the bus viawhich the motor controller sources power to, or sinks power from, themotor/generator).

The motor/generator of FIG. 5 further comprises isolating plates 531,which, in some embodiments, may function to segment the stator iron 530into multiple segments. In such embodiments, the isolating plates 531may function to electrically isolated each segment of the stator iron530 from the other segments. Thus, in the illustrated embodiment, statoriron 530 is split into three electrically-isolated segments 521, 522,and 523. (The stator iron may be split into more or less segments,however, depending upon the particular implementation.) Further, in suchan embodiment, the coils on each segment 521, 522, and 523 may be drivenby a different motor controller. For example, the stator segment 521might be connected to a first motor controller, the stator segment 522to a second motor controller, and stator segment 523 to a third motorcontroller.

In a further aspect of such an embodiment, the bus connections of theseparate motor controllers for segments 521, 522, and 523 may beconnected in series, such that the voltages across the bus connectionsof the motor controllers are additive. For example, if the voltageacross the bus line of each motor controller is 500V, the total voltageacross the set of three motor controllers will be 1500V. When theswitches within the motor controllers are respectively open and exposingthe coils to a bus voltage, the isolation of each individual statorsegment results, and the wire-to-wire voltage difference between anygiven wire in the coils 534 of any single stator segment 521, 522, and523 is always less than or equal to 500V.

In some embodiments, isolating plates 531 fully encapsulate the statorsegments and further act as a corrosion inhibitor or mechanism ofenvironmental isolation, while in other embodiments, the isolatingplates 531 only physically separate the stator segments and the air gapbetween the rotor magnets 533 and the stator 530 is relied on to providevoltage isolation. In some embodiments, the isolating plates 531 provideisolation between stator segments, and a sprayed on coating, for examplea urethane paint, is applied to the surface of the stator segments toprovide further voltage isolation between stator segments. In someembodiments of this invention, other types of motor/generators are used,for example the invention can comprise synchronous, induction,transverse flux, radial flux, axial flux, brushed or brushless typemotors. In some embodiments, each motors/generator comprises isolatedsections, for example the sections 521, 522, and 523, in an axiallystacked configuration. In these embodiments each section is roughlyequivalent of an independent motor/generator, but is mounted on the sameshaft as the other sections.

FIG. 6 is a diagram depicting an embodiment of a kite 601 that may beused in airborne wind turbine, which is similar in design to the kite201 depicted in FIG. 2. In the illustrated example, the airborne windturbine 601 flies primarily in the “x” direction 635, which issubstantially perpendicular to the plane of rotation of the rotors.

In an example embodiment, as gusts or changes in wind can interact withthe kite 601 in a way which yaws or pitches the kite such that the xdirection on the wing points into the apparent wind. At low flightspeeds, the forces created by the lifting surfaces of the kite 601 arelow, and it is desirable to add force to increase the rate at which thekite reorients into the apparent wind. In some embodiments, the kite 601is not stable, and the forces and moments from the aerodynamic surfacesdo not reorient the kite 601 such that the x direction on the wingpoints into the apparent wind. In such embodiments, some added force ormoment may be required in order to maintain stability of the device. Ineither described embodiment or in any other embodiment, separate controlsignals to each rotor may be given (e.g., by the control system 330shown in FIG. 3), resulting primarily in different thrust or drag oneach rotor and thus in pitching moment, a yawing moment, or in anincrease or reducing in drag force. In the present invention, in anexample embodiment, drive units are located such that, if any pair ofdrive units connected in parallel are disabled, control authority andforce balance on each of pitch, yaw, and thrust axis is maintained.

Drive units 623, 627, 626, and 622 are located above the pitch axis orcenter of mass of the kite 601. Drive units 621, 625, 628, and 624 arelocated below the pitch axis or center of mass of the kite 601. Driveunits 621 and 622 are connected electrically in parallel. Each pair ofdrive units 623 and 624, 625 and 626, and 627 and 628 is also connectedin parallel, with each drive-unit pair being connected to the nextdrive-unit pair in series.

As drive units 621 and 622 are located in mirrored locations about thecenter of mass 636, if both drive units are disabled there is nosignificant net torque about the center of mass. In some embodiments,this is useful in allowing for component failures, for example thefailure of a motor controller or a motor/generator, without impact onthe stability of the vehicle.

Each drive unit pair is respectively arranged such that the location ofone drive unit in the pair is a mirrored location of the location of theother drive unit in the pair, about the center of mass of the aerialvehicle. In an example embodiment, the drive units above the wing areeach positioned at substantially the same distance above the wing, andall drive units below the wing are positioned at substantially the samedistance below the wing, and the horizontal spacing of each drive isconsistent. Other arrangements of drive units are possible. For example,the drive units could be located in an octagonal pattern about the rimof a circular wing, or on pylons of different lengths or spacing. Otherexamples are also possible.

In the illustrated example, an increase in power input to drive units623, 627, 626, and 622 by some differential amount dP, and an equivalentdecrease in power input to drive units 621, 625, 628, and 624 by dP doesnot change the relative power usage by each pair of motors but resultsin a pitch-down torque on the kite 601. In the case that any drive pairis disabled, the same balance of power holds. For example, if driveunits 621 and 622 are disabled and the rotors of said drive are freespinning, the remaining drive-unit pairs 623 and 624, 625 and 626, and627 and 628 can apply a pitch down torque that is percentage of theamount that can be applied when all drive units are operable. Further,since each pair of drive units is connected in parallel, each drive-unitpair consumes substantially the same amount of power.

Thus, if all drive-unit pairs are connected in series to a bus ofapproximately fixed voltage, a disabled drive-unit pair will increasethe voltage across each remaining parallel-connected pair. Alternately,in some embodiments, the high voltage DC bus voltage may be controlledon the kite 601 or on the ground at the other end of the tether 603, andthe voltage on said bus may be accordingly be reduced in response todrive-unit pair being disabled, such that the substantially the samevoltage is seen across each drive-unit pair in the series.

In hovering flight, kite 601 is suspended under thrust from all or somedrive units. In an example embodiment, when no torque is output by theset of drive units, equal power is consumed by all drive units. The samemechanism of control as described in the crosswind case is used to applypitching moment and yawing moment to the kite 601.

FIG. 7 is a diagram depicting the thrust and drag curves of a typicalkite rotor. At a given apparent wind velocity on the rotor, there is anadvance ratio which yields no net thrust or drag. At higher advanceratios, the rotor generates torque about the axis of rotation,generating shaft power. At lower advance ratios, the rotor generatestorque counter to the axis of rotation, requiring shaft power tomaintain advance ratio. At high enough advance ratios, the angle ofattack on the blades of the rotor increases near stall, and the powerand drag produced by the rotors no longer increases at higher advanceratios. This effect determines the maximum allowable advance ratio, theadvance ratio at which the rotor no longer produces greater power. Insome embodiments, it is advantageous to operate all rotors near theirmaximum allowable advance ratio so as to maximize power generation at agiven kite speed. The optimization of tether mass and tether dragnaturally leads to high voltages.

Example embodiments may provide a method for interfacing betweencomponents that generate or consume power and the transmission linewhich transmits that power to another location. Specifically, exampleembodiments may apply to situations where there is a mismatch between alower desired power conversion voltage and a higher desired powertransmission voltage. The proposed method can support any non-primenumber of power converters, and encompasses a means of stacking groupsof power converters and the control scheme required for evenly balancingvoltage across the stacks. For airborne wind turbines, the unit of powerconversion is a propeller, motor, and motor controller set.

An example embodiment connects pairs of power converters across thecenter of mass of the wing in parallel and stacks these pairs in series.A control algorithm is then applied to ensure power flow is equalizedacross pairs, ensuring voltage stays within acceptable limits.

An alternative topology applies multiple motors and motor controllersets to each propeller shaft (e.g., such that each drive unit includesmultiple motors and multiple motor controllers). A control algorithm isapplied to ensure even power outtake across motors on the same shaft.The motor controller outputs can then be stacked in series and motorcontrollers from different shafts can be connected in parallel.

The effect of both of the strategies is to stack the output of severallower voltage motor outputs to interface with a higher voltagetransmission element, while at the same time balancing power flow tomaintain acceptable voltages at each stage in the stack.

CONCLUSION

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

We claim:
 1. A system comprising: a plurality of drive units coupled toa plurality of propellers, wherein each drive unit comprises a singlemotor/generator and a single motor controller, and wherein the pluralityof drive units includes a first drive-unit pair and a second drive-unitpair; and a high-voltage bus connecting the motor controllers in thefirst drive-unit pair to a tether, a low-voltage bus connecting themotor controllers in the second drive-unit pair to the tether, and anintermediate-voltage bus connecting the motor controllers of the firstdrive-unit pair in series with the motor controllers of the seconddrive-unit pair; wherein the motor controllers in the first drive-unitpair are connected in parallel via the high-voltage bus and theintermediate-voltage bus; and wherein the motor controllers in thesecond drive-unit pair are connected in parallel via theintermediate-voltage bus and the low-voltage bus.
 2. The system of claim1, further comprising a motor control system operable to send controlsignals to each motor controller, such that each drive unit isseparately controllable.
 3. The system of claim 2, wherein the motorcontrol system is configured to generate control signals that balancethe voltage across the plurality of drive units.
 4. The system of claim3, wherein the motor control system is configured to generate controlsignals for two or more of the motor controllers such that a desiredtorque is applied to an aerial vehicle of the system.
 5. The system ofclaim 2, wherein the motor control system is configured to receive datarelated to the plurality of drive units.
 6. The system of claim 5,wherein the control system is operable to determine the control signalsbased at least in part on the received data related to the plurality ofdrive units.
 7. The system of claim 2, wherein the control system isconfigured to: detect a failure of at least one motor or motorcontroller in a particular pair of drive units; and in response to thefailure, generate control signals for one or more of the motorcontrollers that bypass the particular drive-units pair.
 8. The systemof claim 1, wherein the motor/generators comprise a number of phasewindings, and wherein the motor controllers are configured to sourcepower to the motor/generators at times corresponding to the number ofphase windings.
 9. The system of claim 1, wherein each pair of driveunits comprises a first drive unit arranged at a first location of anaerial vehicle of the system and a second drive unit arranged at asecond location on the aerial vehicle, and wherein, for each pair ofdrive units, the location of the first drive unit mirrors the locationof the second drive unit about a center of mass of the aerial vehicle.10. The system of claim 1, further comprising at least one overrideswitch that is operable to bypass at least one pair of drive units. 11.The system of claim 1, further comprising on-ground equipment adapted toadjust tether voltage when a motor controller fails so as to maintainequal bus voltage over each drive-unit pair.
 12. The system of claim 1,wherein each motor controller is a low-voltage motor controller, whereinthe tether comprises a high-voltage transmission line, and wherein thearrangement of the at least two drive units allows for interfacing ofthe low-voltage motor controllers with the high-voltage transmissionline.
 13. The system of claim 1, wherein the drive units are located onan aerial vehicle of the system such that, if any drive-unit pair of theplurality is disabled, control authority and force balance on each ofpitch, yaw, and thrust axis of the aerial vehicle is maintained.
 14. Asystem comprising: a plurality of drive units coupled to a plurality ofpropellers, wherein each drive unit comprises a single motor/generatorand a single motor controller, and wherein the plurality of drive unitsform a plurality of drive-unit pairs; a tether; a high-voltage busconnecting the motor controllers in a first drive-unit pair of theplurality to the tether; a low-voltage bus connecting the motorcontrollers in a second drive-unit pair of the plurality to the tether;and an intermediate-voltage bus configured to connect the plurality ofdrive-unit pairs in series via the motor controllers in each of theplurality of drive-unit pairs; wherein the motor controllers in thefirst drive-unit pair of the plurality are connected in parallel via thehigh-voltage bus and the at least one intermediate-voltage bus; andwherein the motor controllers in the second drive-unit pair of theplurality are connected in parallel via the at least oneintermediate-voltage bus and the low-voltage bus.
 15. The system ofclaim 14, wherein the plurality of drive units further comprise a thirddrive-unit pair and a fourth drive-unit pair, and wherein theintermediate-voltage bus comprises a first intermediate-voltage bus, asecond intermediate-voltage bus, and a third intermediate-voltage bus,wherein: (i) the first intermediate-voltage bus is configured to connectthe first drive-unit pair in series with the third drive-unit pair viathe motor controllers in the first and third drive-unit pairs, (ii) thesecond intermediate-voltage bus is configured to connect the thirddrive-unit pair in series with the fourth drive-unit pair via the motorcontrollers in the third and fourth drive-unit pairs, (iii) the thirdintermediate-voltage bus is configured to connect the fourth drive-unitpair in series with the second drive-unit pair via the motor controllersin the fourth and second drive-unit pairs, (iv) the first and secondintermediate-voltage buses are configured to connect the motorcontrollers in the third drive-unit pair in parallel, and (v) the secondand third intermediate-voltage buses are configured to connect the motorcontrollers in the fourth drive-unit pair in parallel.
 16. The system ofclaim 14, further comprising a motor control system operable to sendcontrol signals to each motor controller, such that each drive unit isseparately controllable.
 17. The system of claim 16, wherein the motorcontrol system is configured to generate control signals that balancethe voltage across the plurality of drive units.
 18. The system of claim14, wherein an average voltage at the at least one intermediate-voltagebus is approximately half of an incoming voltage on the tether.
 19. Asystem comprising: an aerial vehicle comprising a plurality ofpropellers; a plurality of drive units coupled to the plurality ofpropellers, wherein each drive unit comprises a single motor/generatorand a single motor controller, and wherein the plurality of drive unitsincludes a first drive-unit pair and a second drive-unit pair; and ahigh-voltage bus connecting the motor controllers in the firstdrive-unit pair to a tether, a low-voltage bus connecting the motorcontrollers in the second drive-unit pair to the tether, and anintermediate-voltage bus connecting the motor controllers of the firstdrive-unit pair in series with the motor controllers of the seconddrive-unit pair; wherein the motor controllers in the first drive-unitpair are connected in parallel via the high-voltage bus and theintermediate-voltage bus; and wherein the motor controllers in thesecond drive-unit pair are connected in parallel via theintermediate-voltage bus and the low-voltage bus.
 20. The system ofclaim 19, further comprising a motor control system operable to sendcontrol signals to each motor controller, such that each drive unit isseparately controllable, wherein the motor control system is configuredto apply a control scheme that generates control signals that, for eachdrive unit, evens power outtake across each motor/generator of theplurality of the motor/generators.